(a) Schematic illustration of the spatially separated pump–probe experiment. (b) PEEM image following excitation with 780-nm p-polarized spatially separated femtosecond pulse pairs. (c) PEEM image of the same hemisphere but recorded following excitation with s-polarized laser pulses and probed using p-polarized pulses. Red dashed circles mark the location of the hemisphere and white dashed ovals indicate the approximate positions of the spatially separated pump and probe beams. (d) PEEM images following excitation with 780-nm s-polarized femtosecond pump pulses and p-polarized probe pulses separated by approximately one-half period (1.24 fs). (f) Time trace of electron yield from the red boxed areas in panels (d) and (e) as a function of relative delay between the pulses. The resulting trace demonstrates that the SPP in the upper plane is π radians out-of-phase with that generated in the lower plane.^[78]